Using trackers shows the marketer the effectiveness of an advertising campaign. ySense is an example of a program that uses a tracker:

Whereas "dansbanners" is the campaign name that's appended to the affiliate link along with the "&" character. And supposedly the above link has gotten 25 clicks and 5 signups, the campaign stats will show an increment of 25 clicks and five signups. And the stats will also eventually show how much you've earned from those signups. Whether they've filled out surveys or completed tasks or offers and so forth. Thus demonstrating to the marketer the advertising efficiency of the campaign.

One could also use trackers to determine the cost effectiveness of the advertising. For instance, if $50 was spent promoting a link in a website. And the stats shows that it has gotten 100 clicks, 10 signups and earning $100, generating a $50 net profit. Indicating that it's probably a good campaign to continue with as opposed to one that resulted in a loss.

Thus the importance of using Trackers in Net Marketing!